1. Field of the Invention
The present invention relates to the field of semiconductor processing and more specifically to a method and apparatus for atmospheric and sub-atmospheric processing of a single wafer.
2. Discussion of Related Art
In silicon wafer processing, a wafer undergoes a predetermined sequence and steps to make an electronic circuit. Some steps are carried out at an atmospheric pressure while other steps are carried out at a sub-atmospheric pressure. Typically, a wafer undergoes a process step in a process chamber. Process chambers are loaded by a robot. Either a single robot, or more than one robot, for loading a single process chamber or more than one process chambers together with process chambers is called a tool or platform. Different tools or platforms can contain different of similar process chambers. All tools together contain the necessary process chambers to complete an entire process sequence that is necessary to fabricate an electronic circuit. Wafers are transported from one tool to another tool in cassettes. In each tool a robot takes the wafers out of the cassette and loads them separately or in a batch into a process chamber or multiple process chambers of that particular tool. After processing, the robot returns the wafers to the same cassette or to a different cassette and the entire cassette is then transported to the next tool in the fab to perform the next process step.
In a number of instances, it is advantageous to combine several different process chambers in one tool. In such a tool the robot takes the wafers out of the wafer cassette and loads them into the first process chamber. After the process is finished in that process chamber, instead of returning the wafer to the cassette the robot then loads the wafer into the next process chamber to perform the next process step. After the next process step, there can be another process step and so on until the wafer has undergone all process steps that are available in that tool. After the last process step of that tool, the wafers are then finally returned to their wafer cassette and the cassette transported to the next tool in the fab. Such a tool with one or more different process chambers are presently referred to as “cluster tools”.
The advantages of a cluster tool include: reduced wafer traveling distance, reduced footprint, reduced cycle time, and improved yield. The reduced wafer traveling distance, reduced footprint, and reduced cycle time are a result of the reduced handling of the wafers. The improved yield is a result of the reduced exposure of the wafer surface to the fab atmosphere. The detrimental affect of the fab atmosphere exposure during transport from one tool to another is dependent on the particular sequence of process steps. Fab atmosphere exposure can be very detrimental to electronic circuit yield between certain steps while it may not affect whatsoever the yield between certain other steps.
The clustering of different process steps in one tool also has some disadvantages. For example, if one process chamber is inoperable due to a technical failure, the entire tool may not be available and therefore technical failure in one process chamber can have detrimental affect on the availability of the other process chambers. Nevertheless, in certain occasions, the advantages outlined above of clustering different sequential process tools in one tool might be higher than the disadvantage of lower availability or reliability. Therefore, there are a number of instances where clustering of different process steps and different process chambers around one or more robots in the single tool is desirable. There are a number of examples where this has been done and where commercial success is achieved proving the benefits of such clustering. Most of the existing clustering tools have some process benefit (i.e., reduced exposure to the fab environment increases the yield).
One example of a cluster tool is a sub-atmospheric cluster tool. In such a tool different sub-atmospheric process chambers are provided around a sub-atmospheric wafer handler or robot. In this case, the clustering provides a benefit that the process chambers do not get exposed to the atmosphere and the wafers do not get exposed to the atmosphere while being transferred from one chamber to another chamber. This is especially useful in the sequence, such as titanium nitride sputtering, aluminum sputtering, titanium nitride sputtering which is generally used to form metal interconnects of an integrated circuit. Another example of a cluster tool is an atmospheric process cluster tool. For example, a chemical mechanical polishing process chamber can be clustered with a cleaning step such that the wafers are transported from the chemical polishing process to the cleaning process while the wafers are still in a wet condition. This avoids having to dry the wafers between the two steps. Drying wafers between the two steps makes it much more difficult to clean the wafers.
Thus, what is desired are novel cluster tool combinations as well as cluster tools which utilizes both atmospheric and sub-atmospheric process chambers.